1. Field of the Invention
The present invention relates generally to an apparatus and method for decoding bursts in a mobile communication system. More particularly, the present invention relates to an apparatus and method for decoding bursts in an OFDMA mobile communication system.
2. Description of the Related Art
Generally, in a Wireless Local Area Network (WLAN), a terminal, due to its short range, decreases in performance while on the move or if it goes away from an access point (AP). The wireless Internet based on the 3rd generation (3G) mobile communication system, though it does not have the problems of the WLAN, has a high cost. Wireless Broadband Internet (WiBro), also known as Portable Internet, allows a user to enjoy a high-speed Internet connection any place and any time while on the move, like a mobile phone. WiBro is an intermediate between the wireless Internet and the WLAN, uses a frequency band of 2.3 GHz, and has a transfer speed (i.e. service bandwidth) of about 1 Mbps. The WiBro system is an Orthogonal Frequency Division Multiple Access (OFDMA) mobile communication system based on IEEE 802.16e.
FIG. 1 illustrates a network architecture in the typical OFDMA mobile communication system.
Referring to FIG. 1, the OFDMA mobile communication system includes a Portable Subscriber Station (PSS) 102 serving as a terminal, a Radio Access Station (RAS) 104 serving as a base station (or AP), an Access Control Router (ACR) 106 serving as a base station controller, a Home Agent (HA) 108, and an Authentication, Authorization and Accounting (AAA) server 110. The PSS 102 is an apparatus used by a subscriber to receive portable Internet service. The RAS 104 exchanges data with the PSS 102 via a wireless interface at an end of a wire network, and the ACR 106 controls the PSS 102 and the RAS 104, and routes IP packets. The HA 108 supports IP mobility of the terminal in the home network, and the AAA server 110 permits an access to the portable Internet only for the authorized user, and performs authentication, authorization and accounting on users and devices to provide the portable Internet service. A provider IP network 112 connects the ACR 106 to the HA 108, the AAA server 110 and a common IP network 114.
FIG. 2 illustrates an exemplary frame structure of an OFDMA system using Time Division Duplexing (TDD). It can be seen that a downlink (DL) interval and an uplink (UL) interval are separated in a time domain. A first symbol of a downlink frame is a preamble. A terminal performs synchronization acquisition, Base Station ID acquisition, and channel estimation using the preamble. Because the Base Station ID is used as a seed value for scrambling and subcarrier permutation, the Base Station ID acquisition is necessary to decode DL data bursts. The preamble is followed by a Frame Control Header (FCH) 200, and the FCH 200 includes therein the information necessary for DL-MAP decoding. That is, the FCH 200 contains information on a DL-MAP length and a coding scheme of a DL-MAP. The DL-MAP includes therein the information necessary for DL data burst decoding of the current frame. The included information includes position and size information of each individual burst, and Modulation and Coding Scheme (MCS) information of bursts. Uplink transmission starts from a control symbol, and a guard time used for reducing uplink/downlink transmission time is inserted between the downlink and the uplink at the middle and end of an uplink frame. An IEEE 802.16e-based OFDMA terminal performs a reception process in the manner of measuring a preamble received from the downlink, decoding an FCH burst, performing DL-MAP decoding using DL-MAP information in the decoding result, and decoding general data bursts.
In the OFDMA mobile communication system, an FCH burst is composed of 24-bit information. The 24-bit data constituting the FCH burst is defined by the Medium Access Control (MAC) standard, and includes 8 bits for length information of a DL-MAP, 2 bits for DL-MAP repetition type information, 10 bits for other frame information, and 4 reserved bits, for the currently transmitted frame. According to the 802.16e standard, the 4 reserved bits in the FCH information are fixed to ‘0’.
A coding/decoding process of the FCH burst will be described hereinbelow with reference to FIGS. 3A and 3B. FIGS. 3A and 3B are block diagrams for a description of a coding/decoding process of an FCH burst in a transceiver of a general OFDMA mobile communication system.
Referring to FIG. 3A, 24-bit data to be transmitted from a transmission apparatus to a reception apparatus in the OFDMA mobile communication system is input to a duplicater unit 310. The duplicater unit 310 repeats the 24-bit data twice, and outputs 48-bit data. The reason why the duplicater unit 310 repeats the 24-bit data twice is to match the input bits to 48 bits which are the minimum coding unit in the OFDMA mobile communication system.
The 48-bit data is input to a convolutional coder 320. The convolutional coder 320, having a coding rate of ½, encodes the 48-bit input data and outputs a 96-bit codeword. The 96-bit codeword output from the convolutional coder 320 is input to an interleaver 330 that prevents burst errors. The interleaver 330 interleaves the 96-bit codeword, and outputs the interleaved 96-bit codeword to a repeater 340. The repeater 340 repeats the 96-bit codeword 4 times, and delivers the repeated codeword to a mapper (not shown). The mapper refers to a modulator, and uses one of Quadrature Phase Shift Keying (QPSK), 8-ary Phase Shift Keying (8PSK), 16-ary Quadrature Amplitude Modulation (16QAM), and 64-ary Quadrature Amplitude Modulation (64QAM) according to bit rate. The QPSK modulation scheme is applied to the FCH burst.
Generally, the convolutional coder 320 is a typical channel coder for error correction. The convolutional coder 320 uses a method of defining a mutual relation expression using several bits located before the current bit, and generating a new bit pattern depending on the relation expression. Thus, even though a certain bit suffers an error during transmission, the convolutional decoder 320 detects the defective bit by checking its preceding/following bits, and corrects the detected defective bit. The convolutional coder 320 becomes either a rate-½ convolutional coder or a rate-⅓ convolutional coder according to how many coded bits it outputs for one original input signal bit in generating a new bit pattern. That is, the convolutional coder 320 is called a rate-½ convolutional coder if it outputs 2 coded bits for one input bit, and called a rate-⅓ convolutional coder if it outputs 3 coded bits for one input bit.
FIG. 4 illustrates an exemplary simplified structure of a convolutional coder applied to an OFDMA mobile communication system.
As illustrated in FIG. 4, input bits are sequentially input to 6 cascaded registers 410 to 460 bit by bit on a shift basis at every clock. A first adder 470 adds up an input bit of the first register 410, an output bit of the first register 410, an output bit of the second register 420, an output bit of the third register 430, and an output bit of the last register 460, and outputs a coded bit stream X; A second adder 480 adds up an input bit of the first register 410, an output bit of the second register 420, an output bit of the third register 430, an output bit of the fifth register 450, and an output bit of the last register 460, and generates a coded bit stream Y. Each of the first and second adders 470 and 480, after adding up its inputs, performs a modulo-2 operation on the added value, and outputs a 1-bit result.
Assume that a value of an initial register is ‘00’ and data ‘11010’ is input to a rate-½ convolutional coder. In this case, if the first bit ‘1’ is input, the register output ‘11’, and changes its value to ‘10’. If the next bit ‘1’ is input, the register outputs ‘01’, and changes its value again to ‘11’. By repeating this process, the rate-½ convolutional coder outputs output data ‘1101010010’.
Referring to FIG. 3B, in the reception apparatus, 384 Log Likelihood Ratio (LLR) values for an FCH burst output from a demapper (or demodulator) are input to a combiner 350. The combiner 350 outputs 96 LLR values through 4 combining operations. The 96 LLR values are input to a deinterleaver 360. The deinterleaver 360 deinterleaves the 96 LLR values, and outputs the deinterleaved LLR values to a Viterbi decoder 370. The Viterbi decoder 370 decodes the 96 LLR values, and outputs 48-bit decoded data.
FIG. 5 illustrates an exemplary data structure decoded by a receiver in the general OFDMA mobile communication system.
The 48-bit data decoded by the Viterbi decoder 370 is shown by reference numeral 501, and the 48-bit data that the duplicater unit 310 outputs after repeating 24-bit data twice is shown by reference numerals 502 and 503. The decoded data output from the Viterbi decoder 370 has a twice-repeated (doubled) structure, like the 48-bit data output from the duplicater unit 310.
The FCH burst decoding apparatus having the above repetition structure has a long decoding time, causing an increase in output time of a result value. Because the decoding apparatus has a structure in which the decoding result of the FCH burst is repeated, it cannot obtain performance gain to which additional possible combining is applied.
In addition, a Burst Quality Indicator (BQI) indicating the quality of an FCH burst is not included in the 24-bit FCH information. The BQI means a specific bit based on which after a decoding process of a burst, it can be determined whether decoding of the burst is successful. In a general case, a CRC bit is popularly used as the BQI bit. However, it is not possible to measure the BQI using the coding structure of the FCH burst. That is, the decoded data is obtained by repeating 24-bit data twice as stated in the coding process. Therefore, the BQI is obtained by comparing the repeated 24-bit decoded data. If the comparison result indicates ‘consistency’, it is considered that the FCH burst has undergone successful decoding, and thus has a high quality. However, if the comparison result indicates ‘inconsistency’, it is considered that the FCH burst has failed to undergo successful decoding, and thus has a low quality. The BQI can have a multi-level value, and it is assumed that the BQI has a higher value as the quality is higher. In the OFDMA terminal, after FCH decoding, a quality value is reported to an upper layer, and the upper layer performs an upper algorithm depending on the quality value.
The foregoing method can contribute to performance improvement of the FCH burst and obtain an improved BQI. However, about 50% of the FCH burst is determined to be normal, even though it is actually defective. If the defective FCH burst is determined as a normal FCH burst, the terminal performs decoding on a DL-MAP based on the defective FCH burst. Actually, a CRC bit, which is a BQI bit, is inserted in the DL-MAP. Therefore, if the FCH burst has an error, it is possible to detect occurrence of an error depending on the BQI bit in the DL-MAP. However, the terminal unnecessarily performs the DL-MAP decoding process. In particular, if an error has occurred in DL-MAP length and repetition type information corresponding to DL-MAP information in the FCH burst information, the time and power required for the wrong DL-MAP decoding process greatly increases. For example, an 8-bit value for the DL-MAP length is transmitted as a value of 10. In this case, if the 8th bit, or the Most Significant Byte (MSB), suffers a transmission error from ‘0’ to ‘1’, the DL-MAP length becomes 138, increasing 14 times the time and power required for decoding the information. In addition, when a DL-MAP decoding error occurs, it is not possible to determine whether there is an error in a DL-MAP reception process or in an FCH reception process.
Accordingly, there is a need for an improved apparatus and method for decoding a burst in an OFDMA system.